Transcriptional co-repressor that interacts with nuclear hormone receptors and uses therefor

ABSTRACT

In accordance with the present invention, there are provide novel receptor interacting factors, referred to herein as “SMRT”, i.e., a silencing mediator (co-repressor) for retinoic acid receptor (RAR) and thyroid hormone receptor (TR). SMRT is a novel protein whose association with RAR and TR both in solution and on DNA response elements is destabilized by ligand. The interaction of SMRT with mutant receptors correlates with the transcriptional silencing activities of receptors. In vivo, SMRT functions as a potent co-repressor. A GAL4 DNA binding domain (DBD) fusion of SMRT behaves as a frank repressor of a GAL4-dependent reporter. Together, these data identify a novel class of cofactor which is believed to represent an important mediator of hormone action.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 09/337,384, filed Jun. 21, 1999 now U.S. Pat. No. 6,551,773, which is in turn a divisional application of U.S. patent application Ser. No. 08/522,726, filed on Sep. 1, 1995, now issued as U.S. Pat. No. 6,489,441, the entire contents of each of which are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to intracellular receptors, methods for the modulation thereof, and methods for the identification of novel ligands therefor. In a particular aspect, the present invention relates to methods for the identification of compounds which function as ligands (or ligand precursors) for intracellular receptors. In another aspect, the present invention relates to novel chimeric constructs and uses therefor.

BACKGROUND OF THE INVENTION

A central problem in eukaryotic molecular biology continues to be the elucidation of molecules and mechanisms that mediate specific gene regulation. As part of the scientific attack on this problem, a great deal of work has been done in efforts to identify ligands (i.e., exogenous inducers) which are capable of mediating specific gene regulation. Additional work has been done in efforts to identify other molecules involved in specific gene regulation.

Although much remains to be learned about the specifics of gene regulation, it is known that ligands modulate gene transcription by acting in concert with intracellular components, including intracellular receptors and discrete DNA sequences known as hormone response elements (HREs).

The identification of compounds which directly or indirectly interact with intracellular receptors, and thereby affect transcription of hormone-responsive genes, would be of significant value, e.g., for therapeutic applications.

Transcriptional silencing mediated by nuclear receptors plays an important role in development, cell differentiation, and is directly linked to the oncogenic activity of v-erbA. The mechanism underlying this effect is unknown but is one key to understanding the molecular basis of hormone action. Accordingly, the identification of components involved in transcriptional silencing would represent a great advance in current understanding of mechanisms that mediate specific gene regulation.

Other information helpful in the understanding and practice of the present invention can be found in commonly assigned U.S. Pat. Nos. 5,071,773 and 4,981,784; and U.S. patent application Ser. No. 325,240, filed Mar. 17, 1989 (now issued as U.S. Pat. No. 5,597,693, based on continuation in part application Ser. No. 494,618, filed Mar. 16, 1990); Ser. No. 370,407, filed Jun. 22, 1989 (now issued as U.S. Pat. No. 5,260,432) and Ser. No. 438,757, filed Nov. 16, 1989 (now issued as U.S. Pat. No. 5,091,518), all of which are hereby incorporated herein by reference in their entirety.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention, we have discovered a novel receptor interacting factor, referred to herein as “SMRT”, i.e., a silencing mediator (co-repressor) for retinoic acid receptor (RAR) and thyroid hormone receptor (TR). SMRT is a novel protein whose association with RAR and TR both in solution and on DNA response elements is destabilized by ligand. The interaction of SMRT with mutant receptors correlates with the transcriptional silencing activities of receptors.

In vivo, SMRT functions as a potent co-repressor. A GAL4 DNA binding domain (DBD) fusion of SMRT behaves as a frank repressor of a GAL4-dependent reporter. Together, these data identify a novel class of cofactor which is believed to represent an important mediator of hormone action.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the quantitation by phosphoimager of a dose-dependent dissociation of SMRT from RAR or TR by all-trans retinoic acid (atRA) or thyroid hormone (triiodothyronine or T3).

FIG. 2 presents amino acid (aa) sequences of SMRT (Genbank accession number U37146; SEQ ID NO:1). The aa sequence presented in parentheses (i.e., residues 1330-1376) is an alternatively spliced insert which is not present in the original two-hybrid clone (C-SMRT, aa 981 to C-terminal end). The proline-rich N-terminal domain (aa 1-160) and the glutamine-rich region (aa 1061-1132), as well as the ERDR-rich and SG-rich regions (underlined), are also indicated. The C-terminal region of SMRT (aa 1201 to C-terminal end) shows 48% aa identity to RIP13 (Seol et al., Molecular Endocrinology 9:72-85 (1995)). The rest of the sequence of RIP13 shows 22% aa identity to SMRT (aa 819-1200)

FIG. 3 collectively illustrates mediation of the silencing effect of hRARCα and hTRβ by SMRT in vivo.

FIG. 3(A) illustrates that v-erbA reverses the silencing effect of GAL-RAR (GAL4 DBD-hRARα 156-462) while SMRT restores the silencing effect.

FIG. 3(B) illustrates that the RAR403 truncation mutant reverses the silencing effect of GAL-TR (GAL4 DBD-hTRβ 173-456) while SMRT restores the silencing effect.

FIG. 3(C) illustrates that v-erbA and full length SMRT or C-SMRT have no effect on GAL-VP16 activity.

FIG. 3(D) illustrates that a GAL4 DBD fusion of full length SMRT suppresses the thymidine kinase basal promoter activity containing four GAL4 binding sites. The fold of repression was calculated by dividing the normalized luciferase activity transfected with the GAL4 DBD alone by those transfected with indicated amount of GAL DBD fusion constructs.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, there are provided co-suppressors of steroid/thyroid hormone receptor activity, said co-suppressors having a structure and function characteristic of the silencing mediator for retinoic acid receptor and thyroid hormone receptor.

Co-suppressors contemplated by the present invention have substantially the same sequence as residues 1-1329 plus 1376-1495, as set forth in SEQ ID NO:1, optionally further comprising the amino acid residues set forth in SEQ ID NO:2 (i.e., residues 1330-1375 of SEQ ID NO:1).

The phrase “substantially the same” is used herein in reference to the nucleotide sequence of DNA, the ribonucleotide sequence of RNA, or the amino acid sequence of protein, that have slight and non-consequential sequence variations from the actual sequences disclosed herein. Species that are substantially the same are considered to be equivalent to the disclosed sequences and as such are within the scope of the appended claims. In this regard, “slight and non-consequential sequence variations” mean that sequences that are substantially the same as the DNA, RNA, or proteins disclosed and claimed herein are functionally equivalent to the sequences disclosed and claimed herein. Functionally equivalent sequences will function in substantially the same manner to produce substantially the same compositions as the nucleic acid and amino acid compositions disclosed and claimed herein. In particular, functionally equivalent DNAs encode proteins that are the same as those disclosed herein or that have conservative amino acid variations, such as substitution of a non-polar residue for another non-polar residue or a charged residue for a similarly charged residue. These changes include those recognized by those of skill in the art as those that do not substantially alter the tertiary structure of the protein.

In accordance with another aspect of the present invention, there are provided antibodies raised against the above-described co-suppressor. Such antibodies can be employed for studying tissue localization of invention co-repressor, the structure of functional domains, the purification of receptors, as well as in diagnostic applications, therapeutic applications, and the like. Preferably, for therapeutic applications, the antibodies employed will be monoclonal antibodies.

The above-described antibodies can be prepared employing standard techniques, as are well known to those of skill in the art, using the invention co-repressor or portions thereof as antigens for antibody production. Both anti-peptide and anti-fusion protein antibodies can be used [see, for example, Bahouth et al. (1991) Trends Pharmacol Sci. vol. 12:338-343; Current Protocols in Molecular Biology (Ausubel et al., eds.) John Wiley and Sons, New York (1989)]. Factors to consider in selecting portions of invention co-repressor for use as immunogen (as either a synthetic peptide or a recombinantly produced bacterial fusion protein) include antigenicity, accessibility (i.e., where the selected portion is derived from, e.g., the ligand binding domain, DNA binding domain, dimerization domain, and the like), uniqueness of the particular portion selected (relative to known receptors and co-suppressors therefor), and the like.

In accordance with yet another aspect of the present invention, there are provided methods to block the repressing effect of invention co-suppressors, said method comprising administering an effective amount of an antibody as described herein. Alternatively, a silencing domain of a nuclear receptor can be employed. Those of skill in the art can readily determine suitable methods for administering said antibodies, and suitable quantities for administration, which will vary depending on numerous factors, such as the indication being treated, the condition of the subject, and the like.

In accordance with a still further aspect of the invention, there are provided isolated polynucleic acids encoding the above-described co-suppressor. In addition, there are also provided vectors containing the above-described polynucleic acid.

In accordance with a still further aspect of the present invention, there are provided complexes comprising the above-described co-suppressor and a homodimeric or heterodimeric member of the steroid/thyroid hormone superfamily of receptors, wherein said member contains a silencing domain which represses basal level promoter activity of target genes. Homodimeric or heterodimeric members of the steroid/thyroid hormone superfamily of receptors contemplated for use herein include thyroid hormone receptor homodimer, thyroid hormone receptor-retinoid X receptor heterodimer, retinoic acid receptor homodimer, retinoic acid receptor-retinoid X receptor heterodimer, retinoid X receptor homodimer, and the like.

The above-described complexes optionally further comprise a response element for the member of the steroid/thyroid hormone superfamily of receptors. Such response elements are well known in the art. Thus, for example, RAR response elements are composed of at least one direct repeat of two or more half sites separated by a spacer of five nucleotides. The spacer nucleotides can independently be selected from any one of A, C, G or T. Each half site of response elements contemplated for use in the practice of the invention comprises the sequence -RGBNNM-,

-   -   wherein         -   R is selected from A or G;         -   B is selected from G, C, or T;         -   each N is independently selected from A, T, C, or G; and         -   M is selected from A or C;     -   with the proviso that at least 4 nucleotides of said -RGBNNM-         sequence are identical with the nucleotides at corresponding         positions of the sequence -AGGTCA-. Response elements employed         in the practice of the present invention can optionally be         preceded by N_(x) wherein x falls in the range of 0 up to 5.

Similarly, TR response elements can be composed of the same half site repeats, with a spacer of four nucleotides. Alternatively, palindromic constructs as have been described in the art are also functional as TR response elements.

The above-described co-repressor/dimeric receptor complexes can be dissociated by contacting complex with a ligand for the member of the steroid/thyroid hormone superfamily of receptors.

As employed herein, the term “ligand (or ligand precursor) for a member of the steroid/thyroid hormone superfamily of receptors” (i.e., intracellular receptor) refers to a substance or compound which, in its unmodified form (or after conversion to its “active” form), inside a cell, binds to receptor protein, thereby creating a ligand/receptor complex, which in turn can activate an appropriate hormone response element. A ligand therefore is a compound which acts to modulate gene transcription for a gene maintained under the control of a hormone response element, and includes compounds such as hormones, growth substances, non-hormone compounds that modulate growth, and the like. Ligands include steroid or steroid-like hormone, retinoids, thyroid hormones, pharmaceutically active compounds, and the like. Individual ligands may have the ability to bind to multiple receptors.

Accordingly, as employed herein, “putative ligand” (also referred to as “test compound”) refers to compounds such as steroid or steroid-like hormones, pharmaceutically active compounds, and the like, which are suspected to have the ability to bind to the receptor of interest, and to modulate transcription of genes maintained under the control of response elements recognized by such receptor.

Examples of known ligands include all-trans-retinoic acid (ligand for retinoic acid receptor), 9-cis-retinoic acid (ligand for retinoid X receptor), thyroid hormone (ligand for thyroid hormone receptor), 1,25-dihydroxy vitamin D₃ (ligand for vitamin D₃ receptor), and the like.

In accordance with another aspect of the present invention, there is provided a method to repress the activity of a member of the steroid/thyroid hormone superfamily of receptors containing a silencing domain which represses basal level promoter activity of target genes, said method comprising contacting said member of the steroid/thyroid hormone superfamily of receptors with a sufficient quantity of a co-suppressor as described hereinabove so as to repress the activity of said member. Members of the superfamily contemplated for repression in accordance with this aspect of the present invention include thyroid hormone receptor, retinoic acid receptor, vitamin D receptor, peroxisome proliferator activated receptor, and the like.

In accordance with yet another aspect of the present invention, there is provided a method to identify compounds which relieve the suppression of steroid/thyroid hormone receptor activity caused by a co-suppressor as described hereinabove, said method comprising comparing the size of the above-described co-suppressor/dimeric receptor complex (i.e., complexes comprising the above-described co-suppressor and a homodimeric or heterodimeric member of the steroid/thyroid hormone superfamily of receptors) upon exposure to test compound, relative to the size of said complex in the absence of test compound. An observed size corresponding to intact complex is indicative of an inactive compound, while an observed size that reflects dissociation of the complex is indicative of a compound that disrupts the complex, thereby relieving the suppression caused thereby. Optionally, the complex employed in this assay further comprises a response element for said member of the steroid/thyroid hormone superfamily of receptors.

The size of the above-described complex can readily be determined employing various techniques available in the art. For example, electrophoretic mobility shift assays (EMSA) can be employed (wherein receptor alone or receptor-co-suppressor complex is bound to target DNA and the relative mobility thereof determined). Those of skill in the art can readily identify other methodology which can be employed to determine the size of the complex as a result of exposure to putative ligand.

In accordance with a still further aspect of the present invention, there is provided a method to identify compounds which relieve the suppression of steroid/thyroid hormone receptor activity caused by a co-suppressor as described hereinabove, without substantially activating said receptor, said method comprising:

-   -   comparing the reporter signal produced by two different         expression systems in the absence and presence of test compound,         -   wherein said first expression system comprises a complex             comprising:             -   a homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors                 selected from thyroid hormone receptor homodimer,                 thyroid hormone receptor-retinoid X receptor                 heterodimer, retinoic acid receptor homodimer, or                 retinoic acid receptor-retinoid X receptor heterodimer,             -   a response element for said member of the                 steroid/thyroid hormone superfamily of receptors,                 wherein said response element is operatively linked to a                 reporter, and             -   optionally, invention co-suppressor, and         -   wherein said second expression system comprises a complex             comprising:             -   a homodimeric or heterodimeric form of the same member                 of the steroid/thyroid hormone superfamily of receptors                 as employed in said first expression system, wherein                 said member is mutated such that it retains hormone                 dependent activation activity but has lost its ability                 to repress basal level promoter activity of target                 genes,             -   the same response element-reporter combination as                 employed in said first expression system, and             -   optionally, invention co-suppressor, and thereafter     -   selecting those compounds which provide:         -   a higher reporter signal upon exposure of said compound to             said first expression system, relative to reporter signal in             the absence of said compound, and         -   substantially the same reporter signal upon exposure of said             compound to said second expression system, relative to             reporter signal in the absence of said compound,     -   wherein said selected compounds are capable of relieving the         suppression of steroid/thyroid hormone receptor activity caused         by a co-suppressor having a structure and function         characteristic of the silencing mediator for retinoic acid and         thyroid receptors, but substantially lacking the ability to         activate steroid/thyroid hormone receptor activity.

The addition of invention co-suppressor is optional in the above-described assay because it is present endogenously in most host cells employed for such assays. It is preferred, to ensure the presence of a fairly constant amount of co-suppressor, and to ensure that co-suppressor is not a limiting reagent, that co-suppressor be supplied exogenously to the above-described assays.

Mutant receptors contemplated for use in the practice of the present invention are conveniently produced by expression plasmids, introduced into the host cell by transfection. Mutant receptors contemplated for use herein include RAR403 homodimers, RAR403-containing heterodimers, TR160 homodimers, TR160-containing heterodimers, and the like.

Reporter constructs contemplated for use in the practice of the present invention comprise:

-   -   (a) a promoter that is operable in the host cell,     -   (b) a hormone response element, and     -   (c) a DNA segment encoding a reporter protein,         -   wherein the reporter protein-encoding DNA segment is             operatively linked to the promoter for transcription of the             DNA segment, and         -   wherein the hormone response element is operatively linked             to the promoter for activation thereof.

Hormone response elements contemplated for use in the practice of the present invention are well known in the art, as has been noted previously.

Exemplary reporter genes include chloramphenicol transferase (CAT), luciferase (LUC), beta-galactosidase (β-gal), and the like. Exemplary promoters include the simian virus (SV) promoter or modified form thereof (e.g., ΔSV), the thymidine kinase (TK) promoter, the mammary tumor virus (MTV) promoter or modified form thereof (e.g., ΔMTV), and the like [see, for example, Mangelsdorf et al., in Nature 345:224-229 (1990), Mangelsdorf et al., in Cell 66:555-561 (1991), and Berger et al., in J. Steroid Biochem. Molec. Biol. 41:733-738 (1992)].

As used herein in the phrase “operative response element functionally linked to an operative reporter gene”, the word “operative” means that the respective DNA sequences (represented by the terms “GAL4 response element” and “reporter gene”) are operational, i.e., work for their intended purposes; the word “functionally” means that after the two segments are linked, upon appropriate activation by a ligand-receptor complex, the reporter gene will be expressed as the result of the fact that the “GAL4 response element” was “turned on” or otherwise activated.

In practicing the above-described functional bioassay, the expression plasmid and the reporter plasmid are co-transfected into suitable host cells. The transfected host cells are then cultured in the presence and absence of a test compound to determine if the test compound is able to produce activation of the promoter operatively linked to the response element of the reporter plasmid. Thereafter, the transfected and cultured host cells are monitored for induction (i.e., the presence) of the product of the reporter gene sequence.

Any cell line can be used as a suitable “host” for the functional bioassay contemplated for use in the practice of the present invention. Thus, cells contemplated for use in the practice of the present invention include transformed cells, non-transformed cells, neoplastic cells, primary cultures of different cell types, and the like. Exemplary cells which can be employed in the practice of the present invention include Schneider cells, CV-1 cells, HuTu80 cells, F9 cells, NTERA2 cells, NB4 cells, HL-60 cells, 293 cells, Hela cells, yeast cells, and the like. Preferred host cells for use in the functional bioassay system are COS cells and CV-1 cells. COS-1 (referred to as COS) cells are monkey kidney cells that express SV40 T antigen (Tag); while CV-1 cells do not express SV40 Tag. The presence of Tag in the COS-1 derivative lines allows the introduced expression plasmid to replicate and provides a relative increase in the amount of receptor produced during the assay period. CV-1 cells are presently preferred because they are particularly convenient for gene transfer studies and provide a sensitive and well-described host cell system.

The above-described cells (or fractions thereof) are maintained under physiological conditions when contacted with physiologically active compound. “Physiological conditions” are readily understood by those of skill in the art to comprise an isotonic, aqueous nutrient medium at a temperature of about 37° C.

In accordance with yet another aspect of the present invention, there is provided a method to identify compounds which activate steroid/thyroid hormone receptor activity, but substantially lack the ability to relieve the suppression caused by a co-suppressor as described hereinabove, said method comprising:

-   -   comparing the reporter signal produced by two different         expression systems in the absence and presence of test compound,         -   wherein said first expression system comprises a complex             comprising:             -   a homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors                 selected from thyroid hormone receptor homodimer,                 thyroid hormone receptor-retinoid X receptor                 heterodimer, retinoic acid receptor homodimer, or                 retinoic acid receptor-retinoid X receptor heterodimer,             -   a response element for said member of the                 steroid/thyroid hormone superfamily of receptors,                 wherein said response element is operatively linked to a                 reporter, and             -   optionally, invention co-suppressor, and         -   wherein said second expression system comprises a complex             comprising:             -   a homodimeric or heterodimeric form of the same member                 of the steroid/thyroid hormone superfamily of receptors                 as employed in said first expression system, wherein                 said member is mutated such that it retains hormone                 dependent activation activity but has lost its ability                 to repress basal level promoter activity of target                 genes,             -   the same response element-reporter combination as                 employed in said first expression system, and             -   optionally, invention co-suppressor, and thereafter     -   selecting those compounds which provide:         -   a higher reporter signal upon exposure of said compound to             said second expression system, relative to reporter signal             in the absence of compound, and         -   substantially the same reporter signal upon exposure of said             compound to said first expression system, relative to             reporter signal in the absence of said compound,     -   wherein said selected compounds are capable of activating         steroid/thyroid hormone receptor activity, but substantially         lacking the ability to relieve the suppression caused by a         co-suppressor having a structure and function characteristic of         the silencing mediator for retinoic acid and thyroid receptors.

In accordance with a still further aspect of the present invention, there is provided a method to identify compounds which relieve the suppression of steroid/thyroid hormone receptor activity caused by a co-suppressor as described hereinabove, and activate said receptor, said method comprising:

-   -   comparing the reporter signal produced by two different         expression systems in the absence and presence of test compound,         -   wherein said first expression system comprises a complex             comprising:             -   a homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors                 selected from thyroid hormone receptor homodimer,                 thyroid hormone receptor-retinoid X receptor                 heterodimer, retinoic acid receptor homodimer, or                 retinoic acid receptor-retinoid X receptor heterodimer,             -   a response element for said member of the                 steroid/thyroid hormone superfamily of receptors,                 wherein said response element is operatively linked to a                 reporter, and             -   optionally, invention co-suppressor, and         -   wherein said second expression system comprises a complex             comprising:             -   a homodimeric or heterodimeric form of the same member                 of the steroid/thyroid hormone superfamily of receptors                 as employed in said first expression system, wherein                 said member is mutated such that it retains hormone                 dependent activation activity but has lost its ability                 to repress basal level promoter activity of target                 genes,             -   the same response element-reporter combination as                 employed in said first expression system, and             -   optionally, invention co-suppressor, and thereafter     -   selecting those compounds which provide:         -   increased reporter signal upon exposure of said compound to             said second expression system, relative to reporter signal             in the absence of said compound, and         -   substantially increased reporter signal upon exposure of             said compound to said first expression system, relative to             reporter signal in the absence of said compound,     -   wherein said selected compounds are capable of relieving the         suppression of steroid/thyroid hormone receptor activity caused         by a co-suppressor having a structure and function         characteristic of the silencing mediator for retinoic acid and         thyroid receptors, and activating said receptor.

In accordance with still another embodiment of the present invention, there are provided modified forms of the above-described co-suppressor, including:

-   -   full length silencing mediator for retinoic acid and thyroid         receptors plus GAL4 DNA binding domain,     -   full length silencing mediator for retinoic acid and thyroid         receptors plus GAL4 activation domain,     -   full length silencing mediator for retinoic acid and thyroid         receptors plus glutathione S-transferase (GST) tag,     -   and the like.

The above-described modified forms of invention co-suppressor can be used in a variety of ways, e.g., in the assays described herein.

An especially preferred modified co-suppressor of the invention comprises full length silencing mediator for retinoic acid and thyroid receptors plus GAL4 activation domain.

In accordance with a still further embodiment of the present invention, there is provided a method to identify compounds which disrupt the ability of a co-suppressor as described hereinabove to complex with steroid/thyroid hormone receptors, without substantially activating said receptor, said method comprising:

-   -   comparing the reporter signal produced by two different         expression systems in the absence and presence of test compound,         -   wherein said first expression system comprises a complex             comprising:             -   a modified co-suppressor as described above,             -   a homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors                 selected from thyroid hormone receptor homodimer,                 thyroid hormone receptor-retinoid X receptor                 heterodimer, retinoic acid receptor homodimer or                 retinoic acid receptor-retinoid X receptor heterodimer,                 and             -   a response element for said member of the                 steroid/thyroid hormone superfamily of receptors,                 wherein said response element is operatively linked to a                 reporter, and         -   wherein said second expression system comprises a complex             comprising:             -   said modified co-suppressor,             -   a homodimeric or heterodimeric form of the same member                 of the steroid/thyroid hormone superfamily of receptors                 as employed in said first expression system, wherein                 said member is mutated such that it retains hormone                 dependent activation activity but has lost its ability                 to repress basal level promoter activity of target                 genes, and             -   the same response element-reporter combination as                 employed in said first expression system, and thereafter     -   selecting those compounds which provide:         -   a lower reporter signal upon exposure of said compound to             said first expression system, relative to reporter signal in             the absence of said compound, and         -   substantially the same reporter signal upon exposure of said             compound to said second expression system, relative to             reporter signal in the absence of said compound,     -   wherein said selected compounds are capable of disrupting the         ability of a co-suppressor having a structure and function         characteristic of the silencing mediator for retinoic acid and         thyroid receptors to complex with steroid/thyroid hormone         receptors, without substantially activating said receptor.

Mutant receptors contemplated for use in this embodiment of the present invention include RAR403 homodimers, RAR403-containing heterodimers, TR160 homodimers, TR160-containing heterodimers, and the like.

Suitable host cells for use in this embodiment of the present invention include mammalian cells as well as yeast cells. Yeast cells are presently preferred because they introduce no background since SMRT (i.e., silencing mediator (co-repressor) for retinoic acid receptor (RAR) and thyroid hormone receptor (TR)) is not endogenous to yeast.

In accordance with yet another embodiment of the present invention, there is provided a method to identify compounds which activate steroid/thyroid hormone receptor activity, but substantially lack the ability to disrupt a complex comprising a steroid/thyroid hormone receptor and a co-suppressor as described hereinabove, said method comprising:

-   -   comparing the reporter signal produced by two different         expression systems in the absence and presence of test compound,         -   wherein said first expression system comprises a complex             comprising:             -   a modified co-suppressor as described above,             -   a homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors                 selected from thyroid hormone receptor homodimer,                 thyroid hormone receptor-retinoid X receptor                 heterodimer, retinoic acid receptor homodimer or                 retinoic acid receptor-retinoid X receptor heterodimer,                 and             -   a response element for said member of the                 steroid/thyroid hormone superfamily of receptors,                 wherein said response element is operatively linked to a                 reporter, and         -   wherein said second receptor system comprises:             -   said modified co-suppressor,             -   a homodimeric or heterodimeric form of the same member                 of the steroid/thyroid hormone superfamily of receptors                 as employed in said first expression system, wherein                 said member is mutated such that it retains hormone                 dependent activation activity but has lost its ability                 to repress basal level promoter activity of target                 genes, and             -   the same response element-reporter combination as                 employed in said first expression system, and thereafter     -   selecting those compounds which provide:         -   a higher reporter signal upon exposure of said compound to             said second expression system, relative to reporter signal             in the absence of compound, and         -   substantially the same reporter signal upon exposure of said             compound to said first expression system, relative to             reporter signal in the absence of compound,     -   wherein said selected compounds are capable of activating         steroid/thyroid hormone receptor activity, but substantially         lack the ability to disrupt the complex of a co-suppressor         having a structure and function characteristic of the silencing         mediator for retinoic acid and thyroid receptors and a         steroid/thyroid hormone receptor.

Suitable host cells for use in this embodiment of the present invention include mammalian cells as well as yeast cells. Yeast cells are presently preferred because they introduce no background since SMRT is not endogenous to yeast.

In accordance with a still further embodiment of the present invention, there is provided a method to identify compounds which activate a steroid/thyroid hormone receptor, and disrupt the ability of a co-suppressor as described hereinabove to complex with said receptor, said method comprising:

-   -   comparing the reporter signal produced by two different         expression systems in the absence and presence of test compound,         -   wherein said first expression system comprises a complex             comprising:             -   a modified co-suppressor as described above,             -   a homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors                 selected from thyroid hormone receptor homodimer,                 thyroid hormone receptor-retinoid X receptor                 heterodimer, retinoic acid receptor homodimer or                 retinoic acid receptor-retinoid X receptor heterodimer,                 and             -   a response element for said member of the                 steroid/thyroid hormone superfamily of receptors,                 wherein said response element is operatively linked to a                 reporter, and         -   wherein said second expression system comprises a complex             comprising:             -   said modified co-suppressor,             -   the same homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors as                 employed in said first expression system, wherein said                 member is mutated such that it retains hormone dependent                 activation activity but has lost its ability to repress                 basal level promoter activity of target genes, and             -   the same response element-reporter combination as                 employed in said first expression system, and thereafter     -   selecting those compounds which provide:         -   a reduction in reporter signal upon exposure of compound to             said first expression system, relative to reporter signal in             the absence of said compound, and         -   increased reporter signal upon exposure of compound to said             second expression system, relative to reporter signal in the             absence of said compound,     -   wherein said selected compounds are capable of activating a         steroid/thyroid hormone receptor and disrupting a complex         comprising steroid/thyroid hormone receptor and a co-suppressor         having a structure and function characteristic of the silencing         mediator for retinoic acid and thyroid receptors.

Suitable host cells for use in this embodiment of the present invention include mammalian cells as well as yeast cells. Yeast cells are presently preferred because they introduce no background since SMRT is not endogenous to yeast.

In accordance with yet another aspect of the present invention, there is provided a method to identify compounds which activate a steroid/thyroid hormone receptor and/or disrupt the ability of a co-suppressor as described hereinabove to complex with said receptor, said method comprising:

-   -   comparing the reporter signals produced by a combination         expression system in the absence and presence of test compound,         -   wherein said combination expression system comprises:             -   a first homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors                 selected from thyroid hormone receptor homodimer,                 thyroid hormone receptor-retinoid X receptor                 heterodimer, retinoic acid receptor homodimer, or                 retinoic acid receptor-retinoid X receptor heterodimer,             -   a second homodimeric or heterodimeric form of the same                 member of the steroid/thyroid hormone superfamily of                 receptors as employed in said first homodimer or                 heterodimer, wherein said member is mutated such that it                 retains hormone dependent activation activity but has                 lost its ability to repress basal level promoter                 activity of target genes (i.e., provides basal level                 expression),                 -   wherein either said first homodimer (or heterodimer)                     or said second homodimer (or heterodimer) is                     operatively linked to a GAL4 DNA binding domain,             -   a response element for said member of the                 steroid/thyroid hormone superfamily of receptors,                 wherein said response element is operatively linked to a                 first reporter,             -   a GAL4 response element, wherein said response element                 is operatively linked to a second reporter, and             -   optionally a co-suppressor of steroid/thyroid hormone                 receptor activity, said co-suppressor having a structure                 and function characteristic of the silencing mediator                 for retinoic acid and thyroid receptors, and thereafter     -   identifying as capable of relieving the suppression of         steroid/thyroid hormone receptor activity caused by a         co-suppressor having a structure and function characteristic of         the silencing mediator for retinoic acid and thyroid receptors,         but substantially lacking the ability to activate         steroid/thyroid hormone receptor activity those compounds which         provide:         -   a higher reporter signal from the reporter responsive to the             first member upon exposure of said compound to said first             member, relative to reporter signal in the absence of said             compound, and         -   substantially the same reporter signal from the reporter             responsive to the second member upon exposure of said             compound to said second member, relative to reporter signal             in the absence of said compound, or     -   identifying as capable of activating steroid/thyroid hormone         receptor activity, but substantially lacking the ability to         relieve the suppression caused by a co-suppressor having a         structure and function characteristic of the silencing mediator         for retinoic acid and thyroid receptors those compounds which         provide:         -   a higher reporter signal from the reporter responsive to the             second member upon exposure of said compound to said second             member, relative to reporter signal in the absence of             compound, and         -   substantially the same reporter signal from the reporter             responsive to the first member upon exposure of said             compound to said first member, relative to reporter signal             in the absence of said compound, or     -   identifying as capable of relieving the suppression of         steroid/thyroid hormone receptor activity caused by a         co-suppressor having a structure and function characteristic of         the silencing mediator for retinoic acid and thyroid receptors,         and activating said receptor those compounds which provide:         -   a higher reporter signal from the reporter responsive to the             second member upon exposure of said compound to said second             member, relative to reporter signal in the absence of said             compound, and         -   a greater increase in reporter signal from the reporter             responsive to the first member upon exposure of said             compound to said first member, relative to reporter signal             in the absence of said compound.

Thus, the change in expression level of the two different reporters introduced in a single transfection can be monitored simultaneously. Based on the results of this single transfection, one can readily identify the mode of interaction of test compound with the receptor/SMRT complex.

Exemplary GAL4 response elements are those containing the palindromic 17-mer:

-   -   5′-CGGAGGACTGTCCTCCG-3′ (SEQ ID NO:3),         such as, for example, 17MX, as described by Webster et al., in         Cell 52:169-178 (1988), as well as derivatives thereof.         Additional examples of suitable response elements include those         described by Hollenberg and Evans in Cell 55:899-906 (1988); or         Webster et al. in Cell 54:199-207 (1988).

In accordance with still another embodiment of the present invention, there is provided a method to identify compounds which activate a steroid/thyroid hormone receptor and/or disrupt the ability of a co-suppressor as described hereinabove to complex with said receptor, said method comprising:

-   -   comparing the reporter signals produced by a combination         expression system in the absence and presence of test compound,         -   wherein said combination expression system comprises:             -   a modified co-suppressor as described above,             -   a first homodimeric or heterodimeric member of the                 steroid/thyroid hormone superfamily of receptors                 selected from thyroid hormone receptor homodimer,                 thyroid hormone receptor-retinoid X receptor                 heterodimer, retinoic acid receptor homodimer, or                 retinoic acid receptor-retinoid X receptor heterodimer,             -   a second homodimeric or heterodimeric form of the same                 member of the steroid/thyroid hormone superfamily of                 receptors as employed in said first homodimer or                 heterodimer, wherein said member is mutated such that it                 retains hormone dependent activation activity but has                 lost its ability to repress basal level promoter                 activity of target genes,                 -   wherein either said first homodimer (or heterodimer)                     or said second homodimer (or heterodimer) is                     operatively linked to a GAL4 DNA binding domain,             -   a response element for said member of the                 steroid/thyroid hormone superfamily of receptors,                 wherein said response element is operatively linked to a                 first reporter,             -   a GAL4 response element, wherein said response element                 is operatively linked to a second reporter, and                 thereafter     -   identifying as capable of disrupting the ability of a         co-suppressor having a structure and function characteristic of         the silencing mediator for retinoic acid and thyroid receptors         to complex with a steroid/thyroid hormone receptor, without         substantially activating steroid/thyroid hormone receptor, those         compounds which provide:         -   a lower reporter signal from the reporter responsive to the             first member upon exposure of said compound to said first             member, relative to reporter signal in the absence of said             compound, and         -   substantially the same reporter signal from the reporter             responsive to the second member upon exposure of said             compound to said second member, relative to reporter signal             in the absence of said compound, or     -   identifying as capable of activating steroid/thyroid hormone         receptor activity, but substantially lacking the ability to         disrupt a complex comprising a steroid/thyroid hormone receptor         and a co-suppressor having a structure and function         characteristic of the silencing mediator for retinoic acid and         thyroid receptors, those compounds which provide:         -   a higher reporter signal from the reporter responsive to the             second member upon exposure of said compound to said second             member, relative to reporter signal in the absence of             compound, and         -   substantially the same reporter signal from the reporter             responsive to the first member upon exposure of said             compound to said first member, relative to reporter signal             in the absence of said compound, or     -   identifying as capable of disrupting a complex comprising a         steroid/thyroid hormone receptor and a co-suppressor having a         structure and function characteristic of the silencing mediator         for retinoic acid and thyroid receptors, and activating said         receptor those compounds which provide:         -   a reduction in reporter signal from the reporter responsive             to the first member upon exposure of said compound to said             first member, relative to reporter signal in the absence of             said compound, and         -   increased reporter signal from the reporter responsive to             the second member upon exposure of said compound to said             second member, relative to reporter signal in the absence of             said compound.

In accordance with a still further aspect of the present invention, there is provided a method to identify compounds which relieve the suppression of steroid/thyroid hormone receptor activity caused by a co-suppressor as described hereinabove, said method comprising determining the effect of adding test compound to an expression system comprising:

-   -   a modified member of the steroid/thyroid hormone superfamily of         receptors, wherein said modified member contains an activation         domain which renders said receptor constitutively active,     -   a fusion protein comprising the receptor interaction domain of         SMRT operatively linked to the GAL4 DNA binding domain, and     -   a GAL4 response element operatively linked to a reporter.

Prior to addition of an effective ligand for the member of the steroid/thyroid hormone superfamily of receptors employed herein, the association of the modified member and the fusion protein will be effective to bind the GAL4 response element and activate transcription of the reporter. The presence of an effective ligand is indicated by a reduction of reporter signal upon exposure to ligand, which disrupts the interaction of the modified member and fusion protein.

Activation domains contemplated for use in the practice of the present invention are well known in the art and can readily be identified by the artisan. Examples include the GAL4 activation domain, BP64, and the like.

To summarize, a novel nuclear receptor co-repressor which mediates the transcriptional silencing of RAR and TR has been identified. This discovery is of great interest because transcriptional silencing has been shown to play an important role in development, cell differentiation and the oncogenic activity of v-erbA (Baniahmad et al., EMBO J. 11:1015-1023 (1992)); Gandrillon et al., Cell 49:687-697 (1989)); Zenke et al., Cell 61:1035-1049 (1990); Barlow et al., EMBO J. 13:4241-4250 (1994); Levine and Manley, Cell 59:405-408 (1989); Baniahmad et al., Proc. Natl. Acad. Sci. USA 89:10633-10637 (1992b) and Saitou et al., Nature 374:159-162 (1995)). In fact, v-erbA mutants that harbor the Pro160−>Arg change in the TR neither suppress basal transcription nor are capable of oncogenic transformation (Damm and Evans, (1993) supra)

The function of SMRT as a silencing mediator (co-repressor) of RAR and TR is analogous to mSin3 in the Mad-Max-Sin3 ternary complex (Schreiber-Agus et al., Cell 80:777-786 (1995); and Ayer et al., Cell 80:767-776 (1995)). Because GAL-SMRT functions as a potent repressor when bound to DNA, it is reasonable to speculate that the function of the unliganded receptors is to bring with them SMRT to the template via protein-protein interaction. Thus, the repressor function is intrinsic to SMRT as opposed to the TR or RAR itself (Baniahmad et al., Proc. Natl. Acad. Sci. USA 90:8832-8836 (1993); and Fondell et al., Genes Dev 7:1400-1410 (1993)). It is demonstrated herein that the ligand triggers a dissociation of SMRT from the receptor, which would lead to an initial step in the activation process. This would be followed (or be coincident) with an induced conformational change in the carboxy-terminal transactivation domain (τc, also called AF-2), allowing association with co-activators on the transcription machinery (Douarin et al., EMBO J. 14:2020-2033 (1995); Halachmi et al., Science 264:1455-1458 (1994); Lee et al., Nature 374:91-94 (1995); and Cavailles et al., Proc. Natl. Acad. Sci. USA 91:10009-10013 (1994)). Thus, as has previously been suggested (Damm and Evans (1993) supra), the ligand dependent activation of TR would represent two separable processes including relief of repression and net activation. The isolation of SMRT now provides a basis for dissecting the molecular basis of trans-repression.

The invention will now be described in greater detail by reference to the following non-limiting examples.

EXAMPLE 1 Isolation of SMRT

Using a GAL4 DBD-RXR fusion protein (see, for example, U.S. Ser. No. 08/177,740, incorporated by reference herein in its entirety) as a bait in a yeast two-hybrid screening system (Durfee et al. Genes Dev 7:555-569 (1993)), several cDNA clones encoding receptor interacting proteins were isolated. One of these proteins, SMRT, interacts strongly with unliganded RAR and TR but only weakly with RXR or other receptors in yeast. This protein was selected for further characterization.

EXAMPLE 2 Far-western Blotting Procedure

Total bacteria extracts expressing GST fusions of hRARα (aa 156-462) or hRXRα LBD (aa 228-462) and control extracts expressing GST alone or GST-PML fusion protein were subjected to SDS/PAGE and electroblotted onto nitrocellulose in transfer buffer (25 mM Tris, pH 8.3/192 mM glycine/0.01% SDS). After denaturation/renaturation from 6 M to 0.187 M guanidine hydrochloride in HB buffer (25 mM Hepes, pH 7.7/25 mM NaCl/5 mM MgCl₂/1 mM DTT) filters were saturated at 4° C. in blocking buffer (5% milk, then 1% milk in HB buffer plus 0.05% NONIDET® P-40 (nonylphenol polyoxyethylene ether)). In vitro translated ³⁵S-labeled proteins were diluted into H buffer (20 mM Hepes, pH 7.7/75 mM KCl/0.1 mM EDTA/2.5 mM MgCl₂/0.05% NONIDET® P-40 (nonylphenol polyoxyethylene ether)/1% milk/1 mM DTT) and the filters were hybridized overnight at 4° C. with (1 μM) or without ligand. After three washes with H buffer, filters were dried and exposed for autoradiography or quantitated by phosphoimager.

GST-SMRT is a GST fusion of the C-SMRT encoded by the yeast two hybrid clone. GST-SMRT has been purified, but contains several degradation products.

For yeast two-hybrid screening, a construct expressing the GAL4 DBD-hRXRα LBD (aa 198-462) fusion protein was used to screen a human lymphocyte cDNA library as described (Durfee et al., (1993) supra). Full length SMRT cDNA was isolated from a human HeLa cDNA library (Clontech) using the two-hybrid insert as a probe.

Using the above-described far-western blotting procedure, ³⁵S-labeled SMRT preferentially complexes with bacterial extracts expressing the RAR, marginally associates with RXR and shows no association with control extracts. In contrast, ³⁵S-PPAR selectively associates with its heterodimeric partner, RXR, but not with RAR. In a similar assay, ³⁵S-labeled RAR or TR interacts strongly with SMRT and their heterodimeric partner, RXR, but not with degraded GST products, while ³⁵S-RXR interacts only weakly with SMRT. Binding of ligand to RAR or TR reduces their interactions with SMRT but not with RXR, while binding of ligand to RXR has only slight effect. FIG. 1 shows the quantitation of a dose-dependent dissociation of SMRT from RAR or TR by all-trans retinoic acid (atRA) or thyroid hormone (triiodothyronine or T3), demonstrating that the amount of ligand required for 50% dissociation in both cases are close to the kds for both ligands (Munoz et al. EMBO J. 7:155-159 (1988); Sap et al., Nature 340:242-244 (1989); and Yang et al., Proc. Natl. Acad. Sci. USA 88:3559-3563 (1991)).

Full length SMRT encodes a polypeptide of 1495 amino acids rich in proline and serine residues (see FIG. 2 and SEQ ID NO:1). Genbank database comparison reveals similarity of the C-terminal domain of SMRT to a partial cDNA encoding another receptor interacting protein, RIP13 (Seol et al., (1995) supra), whose role in receptor signaling is unknown. Within this region, there can be identified several potential heptad repeats which might mediate protein-protein interaction with the “a-helical sandwich” structure (Bourguet et al., Nature 375:377-382 (1995)) of the ligand binding domain (LBD) of receptors.

EXAMPLE 3 Characterization of SMRT

Unlike other nuclear receptors, unliganded RAR and TR possess a strong silencing domain which represses basal level promoter activity of their target genes (Damm et al., Nature 339:593-597 (1989); Brent et al., New Biol. 1:329-336 (1989); Baniahmad et al., Cel 61:505-514 (1990); and Baniahmad et al., EMBO J. 11:1015-1023 (1992)). The preferential interaction of SMRT with RAR and TR in the absence of hormone suggests that SMRT may play a role in mediating the transcriptional silencing effect of the receptor.

To further investigate the involvement of SMRT in silencing, the interaction of SMRT with mutant receptors which display distinct silencing and/or transactivation activities was tested as follows. ³⁵S-methionine labeled receptors were used as probes to hybridize immobilized GST-SMRT in the presence (10 μM) or absence of all-trans retinoic acid (atRA). The total bacteria extract expressing GST-RXR was included as a control.

When quantitated by phosphoimager, RAR403 shows a 4-fold better interaction with SMRT than wild type RAR. Both full length RAR or a deletion mutant expressing only the ligand binding domain (LBD, referred to as ΔΔR) associate with SMRT; this association is blocked by ligand.

These results confirm that the LBD alone is sufficient in the interaction. The carboxy-terminal deletion mutant RAR403 is a potent dominant negative suppressor of basal level promoter activity of RAR target genes (Damm et al., Proc. Natl. Acad. Sci. USA 90:2989-2993 (1993); Tsai and Collins, Proc. Natl. Acad. Sci. USA 90:7153-7157 (1993); and Tsai et al., Genes Dev 6:2258-2269 (1992)). As might be predicted from the above studies, RAR403 and its amino terminal deletion derivative, ΔΔR403, interact strongly with SMRT in either the presence or absence of ligand, consistent with SMRT mediating the repressor activity of this mutant.

EXAMPLE 4 Interaction of SMRT with TR Mutants

The interaction of SMRT with two different classes of TR mutants was analyzed next. The first mutant employed is the naturally occurring oncogene, v-erbA, which has strong silencing ability but no transactivation activity (Sap et al., (1989) supra Sap et al., Nature 324:635-640 (1986); Weinberger et al., Nature 318:670-672 (1985); and Weinberger et al., Nature 324:641-646 (1986)). The second mutant employed is a single amino acid change (Pro 160−>Arg) of the rTRa (TR160) which has previously been shown to lose its capacity in basal level suppression but retains hormone dependent transactivation (Thompson et al., Science 237:1610-1614 (1987); and Damm and Evans, Proc. Natl. Acad. Sd. USA 90:10668-10672 (1993)). If SMRT is involved in silencing, it would be expected that SMRT should interact with the v-erbA, but show little or no association with the silencing-defective TR160 mutant.

Interaction of the oncogenic v-erbA and rTRα R160 mutant (TR160) with GST-SMRT was determined in a far-western assay as described above (see Example 2). When quantitated by phosphoimager, the v-erbA shows an 18-fold better interaction with SMRT than hTRβ, and the TR160 mutant shows a 10-fold lower signal than the rTRα.

As one might expect, v-erbA interacts strongly with SMRT both in presence or absence of ligand. In contrast, full length TR160 mutant or LBD of TR160 ΔΔTR160) does not interact significantly with SMRT when compared to the wild type receptor.

These data demonstrate that SMRT plays an important role in mediating transcriptional silencing effects of both RAR and TR. These data also suggest that the release of SMRT from receptors could be a prerequisite step in ligand-dependent transactivation by nuclear receptors.

EXAMPLE 5 Formation of Ternary Complexes Containing SMRT

RAR and TR form heterodimers with RXR, resulting in a complex with high DNA binding ability (Bugge et al., EMBO J. 11:1409-1418 (1992); Yu et al., Cell 67:1251-1266 (1991); and Kliewer et al., Nature 355:446-449 (1992)). Since SMRT interacts with RAR and TR, tests were conducted to determine whether SMRT can also interact with the receptor-DNA complex. Thus, the interaction of SMRT with RXR-RAR heterodimer on a DR5 element (i.e., an AGGTCA direct repeat spaced by five nucleotides) was determined in a gel retardation assay, which is carried out as follows. In vitro translated receptor or unprogrammed reticulocyte lysate (URL) was incubated with 1 μg of poly dIdC on ice for 15 minutes in a total volume of 20 μl containing 75 mM KCl, 7.5% glycerol, 20 mM Hepes (pH 7.5), 2 mM DTT and 0.1% NONIDET® P-40 (nonylphenol polyoxyethylene ether), with or without ligand (in the range of about 10-100 nM employed). A ³²P labeled, double stranded oligonucleotide probe was added into the binding reaction (10,000 cpm per reaction), and the reaction was further incubated for 20 minutes at room temperature. The protein-DNA complex was separated on a 5% native polyacrylamide gel at 150 volts.

SMRT is seen to form a ternary complex with the RXR-RAR heterodimer on a DNA response element in the gel retardation assay. Addition of ligand releases SMRT from this complex in a dose-dependent manner.

Similarly, SMRT is seen to form a ternary complex with the RXR-TR heterodimer on a TR response element; addition of T3 disrupts the formation of this complex.

These data demonstrate that SMRT can be recruited to DNA response elements via protein-protein interaction with RAR or TR in the absence of hormone. Binding of hormone disrupts receptor-SMRT interaction and releases SMRT from the receptor-DNA complex.

EXAMPLE 6 Transient Transfection Assay

CV-1 cells were plated in 24 well plates at a density of 50,000 cells per well. Expression plasmids were transfected into cells by lipofection using DOTAP. In each transfection, 5 ng of GAL-RAR and 15 ng of v-erbA or SMRT were used together with 150 ng of reporter construct containing 4 copies of GAL4 binding sites in front of a minimal thymidine kinase promoter and a CMX-β-gal construct as an internal control. The relative luciferase activity was calculated by normalizing to the β-gal activity.

EXAMPLE 7 Reversal of Transcriptional Silencing

Recently, it has been shown that over expression of RAR or TR could reverse the transcriptional silencing effect of the GAL4 DBD fusion of TR (GAL-TR) or RAR (GAL-RAR) (Baniahmad et al., Mol Cell Biol 15:76-86 (1995); and Casanova et al., Mol Cell Biol. 14:5756-5765 (1994)), co-repressor. A similar effect is observed herein when over expression of v-erbA or RAR403 mutants are shown to reverse the silencing effect of GAL-RAR and GAL-TR on the basal activity of a luciferase reporter (see FIGS. 3A and 3B).

In principle, over expression of SMRT should restore repressor activity when co-expressed with v-erbA or RAR403 competitors. Indeed, results presented in FIG. 3C show that both the full length and the C-terminal domain of SMRT (C-SMRT) can titrate out v-erbA or RAR403 competitor activity and re-endow GAL-RAR and GAL-TR with silencing activity. In contrast, neither v-erbA nor SMRT show any effect on the transactivation activity of GAL-VP16 fusion. Thus, SMRT is able to block the titration effect of v-erbA and RAR403 and functionally replaces the putative co-repressor in this system.

EXAMPLE 8 Direct Recruitment of SMRT to a Heterologous Promoter

If SMRT is the mediator of transcription silencing of TR and RAR by interaction with template-bound unliganded receptors, then direct recruitment of SMRT to a heterologous promoter should result in repression of basal level activity. This was tested by fusing full length SMRT to the GAL4 DBD (GAL-SMRT). The effect of the resulting fusion protein on the activity of the thymidine kinase promoter containing four GAL4 binding sites was analyzed. FIG. 3D shows that GAL-SMRT, like GAL-TR, can silence basal promoter activity in a dose-dependent manner. In contrast, GAL-RXR shows no suppression.

These data suggest that SMRT, when recruited to a promoter by direct DNA binding or via association with an unliganded receptor, functions as a potent transcriptional repressor.

While the invention has been described in detail with reference to certain preferred embodiments thereof, it will be understood that modifications and variations are within the spirit and scope of that which is described and claimed.

SEQUENCE ID NO:1 1 MEAWDAH

DKEAFAAEAQKL

GD

CWTSGL

F

V

REVIKAS

HA

D

51 SAFSYA

GH

L

LGLHDTAR

VL

R

TISN

LISSAKH

SVLERQI 101 GAISQGMSVQLHV

YSEHAKA

VQ

VTMGL

L

MD

KKLA

FSGVKQEQL 151 SPRGQAGPPESLGVPTAQEASVLRGTALGSVPGGSITKGIPSTRVPSDSA 201 ITYRGSITHGTPADVLYKGTITRIIGEDSPSRLDRGREDSLPKGHVIYEG 251 KKGHVLSYEGGMSVTQCSKEDGRSSSGPPHETAAPKRTYDMMEGRVGRAI 301 SSASIEGLMGRAIPPERHSPHHLKEQHHIRGSITQGIPRSYVEAQEDYLR 351 REAKLLKREGTPPPPPPSRDLTEAYKTQALGPLKLKPAHEGLVATVKEAG 401 RSIHEIPREELRHTPELPLAPRPLKEGSITQGTPLKYDTGASTTGSKKHD 451 VRSLIGSPGRTFPPVHPLDVMADARALERACYEESLKSRPGTASSSGGSI 501 ARGAPVIVPELGKPRQSPLTYEDHGAPFAGHLPRGSPVTMREPTPRLQEG 551 SLSSSKASQDRKLTSTPREIAKSPHSTVPEHHPHPISPYEHLLRGVSGVD 601 LYRSHIPLAFDPTSIPRGIPLDAAAAYYLPRHLAPNPTYPHLYPPYLIRG 651 YPDTAALENRQTIINDYITSQQMHHNTATAMAQRADMLRGLSPRESSLAL 701 NYAAGPRGIIDLSQVPHLPVLVPPTPGTPATAMDRLAYLPTAPQPFSSRH 751 SSSPLSPGGPTHLTKPTTTSSSERERDRDRERDRDREREKSILTSTTTVE 801 HAPIWRPGTEQSSGSSGSSGGGGGSSSRPASHSHAHQHSPISPRTQDALQ 851 QRPSVLHNTGMKGIITAVEPSKPTVLRSTSTSSPVRPAATFPPATHCPLG 901 GTLDGVYPTLMEPVLLPKEAPRVARPERPRADTGHAFLAKPPARSGLEPA                                |→C-SMRT 951 SSPSKGSEPRPLVPPVSGHATIARTPAKNLAPHHASPDPPAPPASASDPH 1001 REKTQSKPFSIQELELRSLGYHGSSYSPEGVEPVSPVSSPSLTHDKGLPK 1051 HLEELDKSHLEGELRPKQPGPVKLGGEAAHLPHLRPLPES

PSSSPLL

T 1101 APGVKGH

RVVTLA

HISEVIT

DYTRHHP

LSAPLPAPLYSFPGASCP 1151 VLDLRRPPSDLYLPPPDHGAPARGSPHSEGGKRSPEPNKTSVLGGGEDGI 1201 EPVSPPEGMTEPGHSRSAVYPLLYRDGEQTEPSRMGSKSPGNTSQPPAFF 1251 SK

TESNSA

KSKKQE

NKKLNT

NRNEPEYNISQPGTEIFNMPAITGT 1301 GL

TYRSQA

QEHASTNMGLEAIIRKALM┌GKYDQW.EESPPLSANAFNPL 1350 NASASLPAAMPITAADGRSDHTLTSP┐.GGGGKAKVSGRPSSRKAKSPAPG 1399 LA..SGDRPPSVSSVHSEGDCNRRTPLTNRVWEDRPSSAGSTPFPYNPL

1447 MRLQAG

MASPPPPG

PAGSGP

..AGPHHA...WDEEPKPL

CSQYET

1492 SDSE*  1495 SEQUENCE ID NO:2 GKYDQWEESPPLSANAFNPLNASASLPAAMPITAADGRSDHTLTSP 

1. A method to identify compounds which relieve the suppression of steroid/thyroid hormone receptor activity caused by a co-suppressor having a structure and function characteristic of a silencing mediator for retinoic acid and thyroid hormone receptors, thereby activating said receptor, said method comprising: determining the effect of a test compound on a reporter signal produced by a first expression system and on a reporter signal produced by a second expression system, wherein said first expression system comprises a complex comprising: a homodimeric or heterodimeric member of the steroid/thyroid hormone superfamily of receptors selected from thyroid hormone receptor homodimer, thyroid hormone receptor-retinoid X receptor heterodimer, retinoic acid receptor homodimer, or retinoic acid receptor-retinoid X receptor heterodimer, a response element for said member of the steroid/thyroid hormone superfamily of receptors, wherein said response element is operatively linked to a reporter, and said co-suppressor, wherein said co-suppressor comprises amino acid residues 1-1329 plus 1376-1495, as set forth in SEQ ID NO:1, or conservative amino acid variations thereof and, wherein said co-suppressor forms a complex with at least one member of the stcroi˜Jthyroid hormone superfamily of receptors and wherein said second expression system comprises a complex comprising: a homodimeric or heterodimeric form of the same member of the steroid/thyroid hormone superfamily of receptors as employed in said first expression system, wherein said member is mutated such that it retains hormone dependent activation activity but has lost its ability to repress basal level promoter activity of target genes, the same response element-reporter combination as employed in said first expression system, and said co-suppressor wherein said co-suppressor comprises amino acid residues 1-1329 plus 1376-1495, as set forth in SEQ ID NO:1, or conservative amino acid variations thereof and, wherein said co-suppressor forms a complex with at least one member of the steroid/thyroid hormone superfamily of receptors, and thereafter selecting those compounds which provide: increased reporter signal upon exposure of said compound to said second expression system, relative to reporter signal in the absence of said compound, and increased reporter signal upon exposure of said compound to said first expression system, relative to reporter signal in the absence of said compound, wherein said selected compounds are capable of relieving the suppression of steroid/thyroid hormone receptor activity caused by a co-suppressor of steroid/thyroid hormone receptor activity, said co-suppressor having a structure and function characteristic of the silencing mediator for retinoic acid and thyroid hormone receptors, and activating said receptor.
 2. A method according to claim 1 wherein said mutant receptor is selected from TR160 homodimers or TR160-containing heterodimers.
 3. A method according to claim 1, wherein the expression system is a mammalian or yeast cell.
 4. A method to identify compounds which activate a steroict/thyroid hormone receptor, and disrupt the ability of a co-suppressor having a structure and function characteristic of a silencing mediator for retinoic acid and thyroid receptors to complex with said receptor, said method comprising: determining the effect of a test compound on a reporter signal produced by a first expression system and on a reporter signal produced by a second expression system, wherein said first expression system comprises a complex comprising: a modified co-suppressor, comprising amino acid residues 1-1329 plus 1376-1495 as set forth in SEQ ID NO: 1 or conservative amino acid variations thereof, plus a GAL4 activation domain, wherein said co-suppressor forms a complex with at least one member of the steroid/thyroid hormone superfamily of receptors a homodimeric or heterodimeric member of the steroid/thyroid hormone superfamily of receptors selected from thyroid hormone receptor homodimer, thyroid hormone receptor-retinoid X receptor heterodimer, retinoic acid receptor homodimer or retinoic acid receptor-retinoid X receptor heterodimer, and a response element for said member of the steroid/thyroid hormone superfamily of receptors, wherein said response element is operatively linked to a reporter, and wherein said second expression system comprises a complex comprising: said modified co-suppressor, the same homodimeric or heterodimeric member of the steroid/thyroid hormone superfamily of receptors as employed in said first expression system, wherein said mcanber is mutated such that it retains hormone dependent activation activity but has lost its ability to repress basal level promoter activity of target genes, and the same response element-reporter combination as employed in said first expression system, and thereafter selecting those compounds which provide: a reduction in reporter signal upon exposure of compound to said first expression system, relative to reporter signal in the absence of said compound, and increased reporter signal upon exposure of compound to said second expression system, relative to reporter signal in the absence of said compound, wherein said selected compounds are capable of activating a steroid/thyroid hormone receptor and disrupting a complex comprising steroid/thyroid hormone receptor and a co-suppressor having a structure and function characteristic of the silencing mediator for retinoic acid and thyroid receptors.
 5. A method according to claim 4, wherein the expression system is a mammalian or yeast cell. 